Liquid crystals are a type of substance having different phases of matter, including a phase that exhibits properties between those of a conventional liquid and those of a solid crystal. For instance, a liquid crystal may flows like a liquid while maintaining some of the ordered structure characteristic of crystals. Each of these phases can be distinguished by different optical properties. When viewed under a microscope using a polarized light source, different liquid crystal phases will appear to have distinct textures. The contrasting areas in the textures correspond to domains wherein liquid crystal molecules are oriented in different directions.
One popular application of liquid crystals is liquid crystal displays (LCDs), which rely on the optical properties of certain liquid crystalline substances in the presence or absence of an electric field to generate a display image. In common liquid crystal display devices, the liquid crystal layer sits between two orthogonal polarizers with the liquid crystal alignment chosen so that its relaxed phase is a twisted one. This twisted phase reorients light that has passed through the first polarizer, allowing its transmission through the second polarizer, and further allowing the light to be reflected back to the observer. Under this configuration, the liquid crystal device appears transparent.
When a voltage is applied across the liquid crystal layer, an electric field forms within the liquid crystal layer, and the liquid crystal molecules begin to align parallel to the electric field, gradually untwisting in the center of the liquid crystal layer. In this state, the liquid crystal molecules do not reorient light, so the light polarized at the first polarizer is absorbed at the second polarizer, and the device accordingly loses transparency. The loss of transparency increases with increasing voltage across the liquid crystal layer. As such, voltage applied across the liquid crystal layer can be used switch a pixel between being transparent or being opaque. Color LCD systems follow a similar technique, using color filters used to generate colored pixels.